This invention is directed to an improved container coating composition and in particular to an improved, stable, hydrosol coating composition which forms flexible and ductile finishes on metal.
U.S. Pat. Nos. 3,997,694 and 4,021,396 describe coating compositions which are reported to exhibit properties desirable in metal can manufacturing operations requiring flexibility and adhesion. Attempts to use these coatings in end enamel applications, however, produce coatings which are either too soft or too brittle for the severe forming at the double seam which attaches the end to the can side walls.
End enamels may be classified into two categories: interior and exterior.
Interior enamels are those which provide a barrier to either protect the product being packed from the metal, e.g. beer or delicate fruits, e.g. peaches, or to prevent corrosion of the metal by the product, e.g. corn or tomatoes.
Exterior enamels have three main uses: (1) to provide a slippery surface for high speed filling operations such as those used in the beer industry, (2) to protect inks used for advertising, decoration or instruction, (3) to prevent water spotting of the metal after steam processing or pasteurization.
The coatings are applied to the metal substrate, aluminum, tinplate or tin free steel by direct rollercoating of sheets or coil stock. Generally the film weight will be between 2 and 5 milligrams of dry film per square inch of metal surface. The bake temperature, and time subjected to this temperature will be sufficient to cure the coating.
The metal end is stamped from the sheet or coil and then subjected to various forming operations. The can end may be stamped with stiffening profile rings of various design so that the lightest metals can be used. The can ends are secured to the can walls by a single or double seaming operation. The seaming operation requires excellent flexibility requires excellent flexibility and toughness from the coating so that it will not chip, fracture or peel.
The acrylic/epoxy hydrosol coatings produced in the above-mentioned patents exhibit adequate package stability, but are not useful as end-enamel coatings. Package stability, or shelf-life, is defined as the ability of a coating formulation to maintain relatively constant physical specifications and performance characteristics from the time of initial manufacture to application on a metal substrate. We have now found that a stable composition, suitable for commercial application, can be obtained through the stabilizing effect of a poly (vinyl acetal) resin on an acrylic, epoxy and aminoplast hydrosol. The poly(vinyl acetal) resin is incorporated into the composition during the formation of the acrylic polymers which are similar in composition to those described in the above-mentioned patents.
The improved compositions of this invention utilize a blend of an aminoplast resin in combination with a poly (vinyl acetal)-acrylic co-resin/epoxy hydrosol. The poly (vinyl acetal) resin is present during the formation of the acrylic resin. The nature of its state of combination with the acrylic polymer has not been determined. The improved compositions are package stable and exhibit excellent clarity, hardness, flexibility and adhesion to the metal substrates during the forming operation. It must be emphasized that the poly (vinyl acetal) resin must be present during the formation of the acrylic polymer. Solutioning the vinyl resin after the acrylic resin is prepared produces a co-resin solution which forms an unstable composition when blended with an epoxy and an aminoplast resin.